Incentive therapies designed to reinforce drug abstinence have been effective in enhancing patient motivation and reducing drug use. Recent research suggests particular efficacy for a novel incentive system in which patients earn tokens or vouchers for drug-free urines that are exchanged for retail items or services in the community. We propose a series of four interrelated studies to be conducted in methadone maintenance patients who continue high rates of intravenous cocaine and/or opioid use despite participation in standard methadone treatment. The general objective of these studies is to enhance the effectiveness of voucher-based incentive procedures both for promoting initial abstinence in chronic intravenous polydrug abusers and for preventing relapse. This goal will be accomplished by systematically altering features of the voucher incentive program with attention to behavior analytic principles that suggest how such alternations should influence behavior and by evaluating the result of parametric alteration in controlled clinical research. Study 1 will replicate promising preliminary results showing that voucher incentive programs can have dramatic efficacy in promoting abstinence from supplemental drug use during methadone treatment, and examine the influence of voucher pay amount to determine if treatment effectiveness is improved by increasing the magnitude of the reinforcers employed. Study 2 will determine the influence of voucher scheduling parameters (e.g., "start-up bonus") on rates and patterns of abstinence obtained. Study 3 will determine if relapse can be prevented in previously abstinent patients by using a schedule of intermittent reinforcement that gradually tapers reinforcer delivery over time. Finally, Study 4 will determine if a more intensive intervention program can be effective for impacting drug use of treatment-resistant patients who have failed to become abstinent in a standard voucher incentive program. Results of these interventions will be evaluated on specific primary measures of drug use as well as a broader range of measures of psychosocial functioning and behavioral adjustments needed to achieve abstinence. In all studies, demographic, psychiatric diagnostic and drug use variables will be used to identify characteristics of patients who do and do not respond to voucher incentive therapies. Overall, these studies will provide valuable information concerning the potential of voucher-based incentive therapies to initiate and sustain long-term drug abstinence. Results obtained will improve understanding of the general principles and mechanisms of behavior therapy for intravenous polydrug abusers and should have broad generality for treatment of drug abusers in a variety of settings.